Auto merge of #154122 - Zalathar:no-anon, r=nnethercote

Remove the `anon` query modifier



Prior experiments:
- rust-lang/rust#152268
- rust-lang/rust#153996

[Zulip thread: *Removing the `anon` query modifier*](https://rust-lang.zulipchat.com/#narrow/channel/582699-t-compiler.2Fquery-system/topic/Removing.20the.20.60anon.60.20query.20modifier/with/580760962)

---

There are currently three queries that use the `anon` modifier:
- `check_representability`
- `check_representability_adt_ty`
- `erase_and_anonymize_regions_ty`

It seems that none of them are using `anon` in an essential way.

According to comments and tests, the *representability* queries mainly care about not being eligible for forcing (despite having a recoverable key type), so that if a cycle does occur then the entire cycle will be on the query stack. Replacing `anon` with a new `no_force` modifier gives a modest perf improvement.

The `erase_and_anonymize_regions_ty` query appears to be using `anon` to reduce the dep-graph overhead of a query that is expected to not call any other queries (and thus have no dependencies). Replacing `anon` with either `no_hash` or nothing appears to give only a very small perf hit on `cargo` benchmarks, which is justified by the fact that it lets us remove a lot of machinery for anonymous queries.

We still need to retain some of the machinery for anonymous *tasks*, because the non-query task `DepKind::TraitSelect` still uses it.

---

I have some ideas for a follow-up that will reduce dep-graph overhead by replacing *all* zero-dependency nodes with a singleton node, but I want to keep that separate in case it causes unexpected issues and needs to be bisected or reverted.

Author: bors

src/queries/incremental-compilation-in-detail.md

@@ -418,6 +418,13 @@ deal with all of the above but so far that seemed like more trouble than it woul
 
 ## Query modifiers
 
+> FIXME: Make [`rustc_middle::query::modifiers`] the home for query modifier documentation,
+> and migrate all other useful modifier docs there after verifying that they are still accurate.
+
+[`rustc_middle::query::modifiers`]:
+    https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/query/modifiers/index.html
+
+
 The query system allows for applying [modifiers][mod] to queries.
 These modifiers affect certain aspects of how the system treats the query with
 respect to incremental compilation:
@@ -437,6 +444,9 @@ respect to incremental compilation:
       as an optimization because the system can skip recording dependencies in
       the first place.
 
+ - `no_force` - Never "force" the dep nodes for this query, even if the query's
+   key type is recoverable.
+
  - `no_hash` - Applying `no_hash` to a query tells the system to not compute
    the fingerprint of the query's result.
    This has two consequences:
@@ -475,13 +485,6 @@ respect to incremental compilation:
    For example, it makes no sense to store values from upstream
    crates in the cache because they are already available in the upstream crate's metadata.
 
- - `anon` - This attribute makes the system use "anonymous" dep-nodes for the given query.
-   An anonymous dep-node is not identified by the corresponding query key.
-   Instead, its ID is computed from the IDs of its dependencies.
-   This allows the red-green system to do its change detection even if there is no
-   query key available for a given dep-node -- something which is needed for
-   handling trait selection because it is not based on queries.
-
 [mod]: ../query.html#adding-a-new-kind-of-query